Qualitative and quantitative reconstruction of surface water characteristics and recent hydrographic changes in the Trondheimsfjord, central Norway

In the present study we investigate dinocyst assemblages in the Trondheimsfjord over the last 25 to 50 yr from three
well-dated multi-cores (210Pb and 137Cs)
retrieved along the fjord axis. The downcore distribution of the
cysts is discussed in view of changes of the key surface water
parameters sea-surface temperatures (SSTs) and sea-surface salinities
(SSSs) monitored in the fjord, as well as river discharges. We examine
the impact of the North Atlantic
Oscillation pattern and of waste water supply from the local industry
and agriculture on the fjord ecological state and hence
dinocyst species diversity. Our results show that dinocyst production
and diversity in the fjord is not evidently affected by
human-induced eutrophication. Instead the assemblages appear to be
mainly controlled by the NAO-related changes in
physico-chemical characteristics of the surface mixed layer. Still,
discharges of major rivers were modulated, since 1985 by the
implementation of hydropower plants which certainly influences the
freshwater and nutrient supply into the fjord. The impact,
however, is variable according to the local geographical setting, and
barely differentiated from natural changes in river run
off.

We ultimately test the use of the modern analogue technique (MAT) for
the reconstruction of winter and summer SSTs and SSSs and
annual primary productivity (PP) in this particular fjord setting. The
reconstructed data are compared with time-series of SSTs
and SSSs measured at 10 m water depth, as well as with mean annual PPs
along the Norwegian coast and within Scandinavian
fjords. The reconstructions are in general good agreement with the
instrumental measurements and observations from other
fjords. Major deviations can be addressed to peculiarities in the
assemblages linked to the particular fjord setting and the
related hydrological structure.